The present invention relates generally to semiconductor technology and, more particularly, to a method and apparatus for the practice of atomic layer deposition.
In the manufacture of integrated circuits, many methods are known for depositing and forming various layers on a substrate. Chemical vapor deposition (CVD) and its variant processes are utilized to deposit thin films of uniform and, often times conformal coatings over high-aspect and uneven features present on a wafer. However, as device geometries shrink and component densities increase on a wafer, new processes are needed to deposit ultrathin film layers on a wafer. The standard CVD techniques have difficulty meeting the uniformity and conformity requirements for much thinner films.
One variant of CVD to deposit thinner layers is a process known as atomic layer deposition (ALD). ALD has its roots originally in atomic layer epitaxy, which is described in U.S. Pat. Nos. 4,058,430 and 4,413,022 and in an article titled “Atomic Layer Epitaxy” by Goodman et al.; J. Appl. Phys. 60(3), 1 Aug. 1986; pp. R65-R80. Generally, ALD is a process wherein conventional CVD processes are divided into single-monolayer depositions, wherein each separate deposition step theoretically reaches saturation at a single molecular or atomic monolayer thickness or less and, then, self-terminates.
The deposition is an outcome of chemical reactions between reactive molecular precursors and the substrate (either the base substrate or layers formed on the base substrate). The elements comprising the film are delivered as molecular precursors. The desired net reaction is to deposit a pure film and eliminate “extra” atoms (molecules) that comprise the molecular precursors (ligands). In a standard CVD process, the precursors are fed simultaneously into the reactor. In an ALD process, the precursors are introduced into the reactor separately, typically by alternating the flow, so that only one precursor at a time is introduced into the reactor. For example, the first precursor could be a metal precursor containing a metal element M, which is bonded to an atomic or molecular ligand L to form a volatile molecule MLx. The metal precursor reacts with the substrate to deposit a monolayer of the metal M with its passivating ligand. The chamber is purged and, then, followed by an introduction of a second precursor. The second precursor is introduced to restore the surface reactivity towards the metal precursor for depositing the next layer of metal. Thus, ALD allows for single layer growth per cycle, so that much tighter thickness controls can be exercised over standard CVD process. The tighter controls allow for ultrathin films to be grown.
In practicing CVD, a nucleation step is assumed when a film of stable material is deposited on a stable substrate. Nucleation is an outcome of only partial bonding between the substrate and the film being deposited. Molecular precursors of CVD processes attach to the surface by a direct surface reaction with a reactive site or by CVD reaction between the reactive ingredients on the surface. Of the two, the CVD reaction between the reactive ingredients is more prevalent, since the ingredients have much higher affinity for attachment to each other. Only a small fraction of the initial film growth is due to direct surface reaction.
An example of nucleation is illustrated in FIGS. 1-3. FIG. 1 shows a substrate 10 having bonding locations 11 on a surface of the substrate. Assuming that the CVD reaction involves a metal (M) and a ligand (Lx) reacting with a non-metal (A) and hydrogen (Hz), the adsorbed species diffuse on the surface and react upon successful MLx—AHz collisions. However, the reaction does not occur at all of the potential attachment (or bonding) locations 11. Generally, defect sites (sites having irregular topology or impurity) are likely to trap molecular precursors for extended times and, therefore, have higher probability to initiate nucleation. In any event, as shown in FIG. 1, the bonding of the precursor to the surface occurs at only some of the bonding locations 12.
Subsequently, as shown in FIG. 2, the initial bonding sites 12 commence to further grow the thin film material on the surface of the substrate 10. The initial reaction products on the surface are the nucleation seed, since the attached products are immobile and diffusing molecular precursors have a high probability to collide with them and react. The process results in the growing of islands 13 on the substrate surface together with the continuous process of creating new nucleation sites 14. However, as the islands 13 grow larger, the formation of new nucleation seeds is suppressed because most of the collisions occur at the large boundaries of the islands 13.
As the islands 13 enlarge three-dimensionally, most of the adsorption and reaction processes occur on the island surfaces, especially along the upper surface area of the islands 13. Eventually, this vertical growth results in the islands becoming grains. When the grains finally coalesce into a continuous film, the thickness could be on the order of 50 angstroms. However, as shown in FIG. 3, the separated nucleation sites can result in the formation of grain boundaries and voids 15 along the surface of the substrate, where potential bonding sites failed to effect a bond with the precursor(s). The grain boundaries and voids 15 leave bonding gaps along the surface of the substrate so that substantial film height will need to be reached before a continuous upper surface of the film layer is formed.
Although the results described above from nucleation is a problem with the standard CVD process, the effect is amplified with ALD. Since ALD utilizes one precursor at a time, the initial bonding will occur due to surface reaction of the initial precursor with sparse surface defects. Accordingly, seed nucleation sites 12 are very sparse (more sparse than CVD) and nucleation proceeds by growing ALD layers on these few seed sites. As a result, the nuclei grow three-dimensional islands 13 and coalesce only at thickness that are comparable to the distance between the nucleation seeds. That is, the voids 15 could be much larger in size, so that a much higher structure is needed to provide a continuos upper surface for the film when only ALD is used.
Accordingly, if an ALD film can initiate growth on a substrate predominantly by nucleation, the film grows discontinuously for a much thicker distance. Ultimately a much thicker film is practically needed in the case of ALD to achieve continuous film, than that which can be obtained from CVD processes.
The present invention is directed to providing a technique to deposit ALD thin films of reduced thickness that has continuous interface and film.